Separator for battery, method for manufacturing the same, and lithium secondary battery

Abstract

The separator for a battery according to the present invention is a separator for a battery including an insulator layer containing a fibrous material having a heat resistant temperature of equal to or higher than 150° C., insulating inorganic fine particles and a binder, or a separator for a battery including a porous layer formed of a thermal melting resin and an insulator layer containing insulating inorganic fine particles and a binder, wherein water content per unit volume is equal to or smaller than 1 mg/cm 3 when the separator is held for 24 hours in an atmosphere with a relative humidity of 60% at 20° C. The use of the separator for a battery according to the present invention makes it possible to provide a lithium secondary battery that has favorable reliability and safety and is excellent in storage characteristics and charge-discharge cycle characteristics.

The invention claimed is: 
     
       1. A separator for a battery, comprising:
 a porous film formed of a thermal melting resin; and 
 an insulator layer formed of insulating inorganic fine particles and a binder, 
 wherein the separator has the insulator layer on the porous film, 
 the thermal melting resin includes at least one selected from the group consisting of polyethylene, copolyolefin whose structural unit derived from ethylene is at least 85 mol %, and polypropylene, 
 the separator has a porosity in a range of 20% to 70%, 
 the separator has a thickness of 3 μm or more and 20 μM or less, and 
 water content per unit volume is equal to or smaller than 1 mg/cm 3  when the separator is held for 24 hours in an atmosphere with a relative humidity of 60% at 20° C. 
 
     
     
       2. The separator for a battery according to  claim 1 , wherein the inorganic fine particles have an average particle diameter larger than 1/100 of a thickness of the separator. 
     
     
       3. The separator for a battery according to  claim 1 , wherein surfaces of the inorganic fine particles are treated to be hydrophobic. 
     
     
       4. The separator for a battery according to  claim 3 , wherein the surfaces of the inorganic fine particles are treated to be hydrophobic with at least one kind of a surface reforming agent selected from the group consisting of silazane, a silane coupling agent, a silicone oil, a titanate coupling agent, an aluminate coupling agent and a zirconate coupling agent. 
     
     
       5. The separator for a battery according to  claim 1 , wherein the inorganic fine particles are silica, alumina or boehmite treated with heat. 
     
     
       6. The separator for a battery according to  claim 1 , wherein the inorganic fine particles comprise platy particles. 
     
     
       7. The separator for a battery according to  claim 1 , wherein surfaces of the inorganic fine particles are heat-treated to be hydrophobic. 
     
     
       8. The separator for a battery according to  claim 1 , wherein the thermal melting resin has a melting temperature of 80° C. to 140° C. 
     
     
       9. The separator for a battery according to  claim 1 , wherein the inorganic fine particles have an average particle diameter in a range of 0.1 μm to 5 μm. 
     
     
       10. The separator for a battery according to  claim 1 , wherein the amount of the inorganic fine particles contained in the separator is in a range of 20% to 70% by volume in a total volume of the constituent components of the separator. 
     
     
       11. The separator for a battery according to  claim 1 , wherein the inorganic fine particles are oxide. 
     
     
       12. The separator for a battery according to  claim 1 , wherein the separator has an air permeability of 10 to 300 seconds on the basis of a Gurley value. 
     
     
       13. The separator for a battery according to  claim 1 , wherein the water content is equal to or smaller than 0.5 mg/cm 3 . 
     
     
       14. A lithium secondary battery comprising:
 a negative electrode; 
 a positive electrode; 
 an organic electrolyte solution; and 
 the separator for a battery according to  claim 1 . 
 
     
     
       15. The separator for a battery according to  claim 1 , wherein the binder includes at least one selected from the group consisting of an ethylene-vinyl acetate copolymer with a structural unit derived from vinyl acetate of 20 to 35 mol %, an ethylene-acrylate copolymer, a cross-linked substance of polyacrylate, a fluorine-based rubber, a styrene-butadiene rubber, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyurethane, and an epoxy resin. 
     
     
       16. A lithium secondary battery comprising a negative electrode, a positive electrode, an organic electrolyte solution, and a separator for a battery,
 wherein the separator comprises a porous film formed of a thermal melting resin, and an insulator layer formed of insulating inorganic fine particles and a binder, 
 the separator has the insulator layer on the porous film, 
 the thermal melting resin includes at least one selected from the group consisting of polyethylene, copolyolefin whose structural unit derived from ethylene is at least 85 mol %, and polypropylene, 
 the separator has a porosity in a range of 20% to 70%, 
 the organic electrolyte solution is a nonaqueous solution containing an organic solvent and a lithium salt, and 
 a total water concentration of the organic electrolyte solution is equal to or lower than 500 ppm. 
 
     
     
       17. The lithium secondary battery according to  claim 16 , wherein surfaces of the inorganic fine particles are treated to be hydrophobic. 
     
     
       18. The lithium secondary battery according to  claim 17 , wherein the surfaces of the inorganic fine particles are treated to be hydrophobic with at least one kind of a surface reforming agent selected from the group consisting of silazane, a silane coupling agent, a silicone oil, a titanate coupling agent, an aluminate coupling agent and a zirconate coupling agent. 
     
     
       19. The lithium secondary battery according to  claim 16 , wherein the inorganic fine particles are silica, alumina or boehmite treated with heat. 
     
     
       20. The lithium secondary battery according to  claim 16 , wherein the inorganic fine particles comprise platy particles. 
     
     
       21. The lithium secondary battery according to  claim 16 , wherein surfaces of the inorganic fine particles are heat-treated to be hydrophobic. 
     
     
       22. The lithium secondary battery according to  claim 16 , wherein the thermal melting resin has a melting temperature of 80° C. to 140° C. 
     
     
       23. The lithium secondary battery according to  claim 16 , wherein the inorganic fine particles have an average particle diameter in a range of 0.1 μm to 5 μm. 
     
     
       24. The lithium secondary battery according to  claim 16 , wherein the separator has a thickness of 3 μm or more and 20 μm or less. 
     
     
       25. The lithium secondary battery according to  claim 16 , wherein the amount of the inorganic fine particles contained in the separator is in a range of 20% to 70% by volume in a total volume of the constituent components of the separator. 
     
     
       26. The lithium secondary battery according to  claim 16 , wherein the inorganic fine particles are oxide. 
     
     
       27. The lithium secondary battery according to  claim 16 , wherein the separator has an air permeability of 10 to 300 seconds on the basis of a Gurley value. 
     
     
       28. The lithium secondary battery according to  claim 16 , wherein the binder includes at least one selected from the group consisting of an ethylene-vinyl acetate copolymer with a structural unit derived from vinyl acetate of 20 to 35 mol %, an ethylene-acrylate copolymer, a cross-linked substance of polyacrylate, a fluorine-based rubber, a styrene-butadiene rubber, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, polyurethane, and an epoxy resin. 
     
     
       29. The lithium secondary battery according to  claim 16 , wherein the inorganic fine particles have an average particle diameter larger than 1/100 of a thickness of the separator.